Post-Operatively Adjustable Spinal Fixation Devices

ABSTRACT

A system for spinal fixation with a non-rigid portion at least one of the caudal or cephalad terminus. Various devices and techniques are described for transition from a rigid fixation construct to a less rigid support structure applied to a “soft zone” that will help share the stress created on the spinal levels caused by the fixed levels below. In specific embodiments the soft zone is provided by terminating the construct with one of a flexible tether or a dampening rod.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International ApplicationPCT/US17/17700, filed 13 Feb. 2017 (pending). International ApplicationPCT/US17/17700 cites the priority of U.S. Patent Application No.62/294,975, filed on 12 Feb. 2016 (expired). The contents of both of theforegoing applications are incorporated herein in their entireties.

BACKGROUND

Field

The present disclosure relates generally to medical devices, andspecifically to surgical instruments and methods for performing spinalprocedures.

Background

The spine is critical in human physiology for mobility, support, andbalance. The spine protects the nerves of the spinal cord, which conveycommands from the brain to the rest of the body, and convey sensoryinformation from the nerves below the neck to the brain. Even minorspinal injuries can be debilitating to the patient, and major spinalinjuries can be catastrophic. The loss of the ability to bear weight orpermit flexibility can immobilize the patient. Even in less severecases, small irregularities in the spine can put pressure on the nervesconnected to the spinal cord, causing devastating pain and loss ofcoordination.

The spinal column is a bio-mechanical structure composed primarily ofligaments, muscles, bones, and connective tissue that forms a series ofvertebral bodies stacked one atop the other and intervertebral discsbetween each vertebral body. The spinal column provides support to thebody and provides for the transfer of the weight and the bendingmovements of the head, trunk and arms to the pelvis and legs; complexphysiological motion between these parts; and protection of the spinalcord and the nerve roots.

The stabilization of the vertebra and the treatment for spinalconditions is often aided by a surgically implanted fixation devicewhich holds the vertebral bodies in proper alignment and reduces thepatient's pain and prevents neurologic loss of function. Spinal fixationis a well-known and frequently used medical procedure. Spinal fixationsystems are often surgically implanted into a patient to aid in thestabilization of a damaged spine or to aid in the correction of otherspinal deformities. Existing systems often use a combination of rods,plates, pedicle screws, bone hooks, locking screw assemblies, andconnectors, for fixing the system to the affected vertebrae. The systemcomponents may be rigidly locked together to fix the connected vertebraerelative to each other, stabilizing the spine until the bones can fusetogether.

Whatever the treatment, the goal remains to improve the quality of lifefor the patient. In the vast majority of cases this goal is achieved,however in some instances patients who receive implants to treat theprimary pathology develop a secondary condition called junctionaldisease. Most commonly this occurs at the proximal or cephalad area ofspinal instrumentation and is then termed “adjacent segment pathology.”Clinical Adjacent Segment Pathology (CASP) refers to clinical symptomsand signs related to adjacent segment pathology. Radiographic AdjacentSegment Pathology (RASP) refers to radiographic changes that occur atthe adjacent segment. A subcategory of CASP and RASP that occurs at theproximal end of the instrumentation is termed proximal junctionalkyphosis (PJK). PJK may be defined in several manners and commonly isspecified as kyphosis measured from one segment cephalad to the upperend instrumented vertebra to the proximal instrumented vertebra withabnormal value defined as 10° or greater. In practice this often meansthat the patient's head and/or shoulders tend to fall forward to agreater degree than should normally occur. Sometimes the degree issignificant.

Adjacent segment pathology can occur as either a degenerative, traumaticor catastrophic condition and sometimes as a result from a combinationof factors. Degenerative conditions are ones that occur over a period oftime, normally 5 or 6 years but can occur at an accelerated rateparticularly with altered mechanics related to spinal fusion. As aresult the patient's head and/or shoulder region(s) fall forwardgradually over time. Traumatic and catastrophic conditions occur as agenerally sudden shifting of the vertebral body immediately cephalad tothe upper end instrumented vertebra and can lead to sudden changes inspinal alignment with marked symptoms noted by the patient.

Whether the condition is degenerative, traumatic, or catastrophic, theexact cause of adjacent segment pathology is uncertain. Without wishingto be bound by any hypothetical model, it is generally believed thatadjacent segment pathology and more specifically PJK is a result ofexcess strain and stress on the proximal instrumented spinal segmentwhich is then at least partially transferred to the bone structures,disc, ligaments and other soft tissues, causing a loss of normalstructural integrity and mechanical properties. The resultant effect canbe a forward (i.e. kyphotic) shift of the adjacent non-instrumentedvertebral body. One such theory is that this strain and stress is causedby suboptimal alignment and/or balance of the screw and rod construct.Another theory is that the rigidity of the screw and rod constructcauses the problem in that the transition from a motion-restrainedsegment to a motion-unrestrained segment is too much for thenon-instrumented (unrestrained) segment to handle over time. Yet anothertheory speculates that the specific level at which the proximalinstrumented vertebra is located is of vital importance in that somelevels may be better suited to handle a proximal termination of afixation construct than others.

Thus there remains a need for continued improvements and new systems forspinal fixation with a specific goal of preventing the occurrence of orreducing the degree of adjacent segment pathology and failures occurringat either the distal junction (DJK) or proximal junction (PJK). Theimplants and techniques described herein are directed towards overcomingthese challenges and others associated with posterior spinal fixation.

SUMMARY

The problems noted above, as well as potentially others, are addressedin this disclosure by a system for spinal fixation with a non-rigidportion at least one of the caudal or cephalad terminus. Various devicesand techniques are described for transition from a rigid fixationconstruct to a less rigid support structure applied to a “soft zone”that will help share the stress created on the spinal levels caused bythe fixed levels below. In specific embodiments the soft zone isprovided by terminating the construct with one of a flexible tether or adampening rod.

In a first aspect, a system for spinal fixation is provided comprising:a first bone anchor, anchored to a first vertebra in a subject, thefirst bone anchor comprising a first bone fastener attached to a firstrod housing; a rigid spinal rod seated in the first rod housing torestrict translation of the rigid spinal rod relative to the first boneanchor; a second bone anchor, anchored to a second vertebra in thesubject, the second bone anchor comprising a second bone fastenerattached to a second rod housing, wherein the rigid spinal rod is seatedin the second rod housing to restrict translation of the rigid spinalrod relative to the second bone anchor; and a compressible spinalconnector, connected to the second bone anchor, and anchored to a thirdvertebra in the subject, the compressible spinal connector comprising amodulation mechanism for modulating at least one of the tension on thecompressible spinal connector or the resistance to compression of thecompressible spinal connector, wherein said modulation occurs inresponse to a remote signal.

In a second aspect, a spinal tether assembly for providing non-rigidintervertebral support is provided, comprising: a flexible tether; andan adjustable tensioner connected to exert tension on the flexibletether, the adjustable tensioner comprising a first magnet mounted torotate in response to a spinning magnetic field; and a tensioningmechanism configured to convert rotation of the magnet to a decrease orincrease of tension on the flexible tether, depending on the directionof the first magnet's rotation.

In a third aspect, a dampening spinal rod to adjust friction againsttension and compression is provided, comprising: an elongate rigidportion for insertion into a bone anchor; a flared portion for receivinga terminal end of a second spinal rod, the flared portion comprising arod cavity of sufficient diameter to accept the second spinal rod, and afriction control mechanism configured to modulate friction between thesecond spinal rod and said dampening spinal rod in response to a remotesignal.

In a fourth aspect, a method of fixing the relative positions of a firstvertebra and a second vertebra in a subject is provided, the methodcomprising: anchoring a first bone anchor to the first vertebra, thefirst bone anchor comprising a first bone fastener attached to a firstrod housing; seating a rigid spinal rod in the first rod housing torestrict translation of the rigid spinal rod relative to the first boneanchor; anchoring a second bone anchor to the second vertebra, thesecond bone anchor comprising a second bone fastener attached to asecond rod housing, seating the rigid spinal rod in the second rodhousing to restrict translation of the rigid spinal rod relative to thesecond bone anchor; connecting a compressible spinal connector to thesecond bone anchor, the compressible spinal connector comprising amodulation mechanism for modulating at least one of the tension on thecompressible spinal connector or the resistance to compression of thecompressible spinal connector, wherein said modulation occurs inresponse to a remote signal; anchoring the compressible spinal connectorto a third vertebra in the subject; and transmitting the remote signalto the modulation mechanism post-operatively, to cause said modulationto occur.

The above presents a simplified summary in order to provide a basicunderstanding of some aspects of the claimed subject matter. Thissummary is not an extensive overview. It is not intended to identify keyor critical elements or to delineate the scope of the claimed subjectmatter. Its sole purpose is to present some concepts in a simplifiedform as a prelude to the more detailed description that is presentedlater.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. An example of a spinal fixation system.

FIG. 2. A side view of an embodiment of the dampening spinal rod as partof a larger fixation system, showing only the skeletal system of thesubject.

FIG. 3. A dorsal (anterior) view of an embodiment of the spinal fixationsystem's terminal region having multiple soft connections.

FIG. 4. A perspective view of an embodiment of a tensioner that modulesthe tension on a flexible tether.

FIG. 5. An embodiment of a locking mechanism for locking a tensioner.

FIG. 6. A cross-sectional view of an embodiment of a telescoping spinalrod.

FIG. 7. An embodiment of a turnbuckle tensioner for modulating thetension on a flexible tether.

FIG. 8. A dorsal (anterior) view of an embodiment of the spinal fixationsystem's terminal region having multiple soft connections.

FIG. 9. A top view of an embodiment of the external control device.

DETAILED DESCRIPTION

Illustrative embodiments of a system for spinal fixation, parts, andmethods for use thereof, are described below. In the interest ofclarity, not all features of an actual implementation are described inthis specification. It will of course be appreciated that in thedevelopment of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure. The system for spinal fixation, parts, and methods foruse thereof disclosed herein boasts a variety of inventive features andcomponents that warrant patent protection, both individually and incombination.

This disclosure describes a variety of transitional or terminalcomponents that may be implanted as part of a spinal fixation construct5 to decrease the potential for subsequent development of junctionaldisease or failure. In the examples shown only the cephalad most level(for terminal hardware) or levels (for multilevel transitional hardware)of the fixation construct 5 (e.g. those utilizing the exemplarycomponents described herein) are illustrated. It should be appreciated,however, that the entire fixation construct 5 may extend any number oflevels from a single level construct to a long construct spanningmultiple spinal levels and multiple spinal regions from the lumbosacralto cervical regions (such as the example construct illustrated in FIG.1), and with any variety of combinations of known anchors, rods, andconnectors. It should also be appreciated that the exemplary terminaland/or transitional components may additionally or alternatively beutilized at the caudal end of the fixation construct. Moreover, althoughthe vertebral fixation systems 5 described herein may be used along anyaspect of the spine (e.g. anterior, posterior, antero-lateral,postero-lateral) they are particularly suited for implantation along aposterior aspect of the spine.

A general embodiment of the system comprises a first bone anchor 10,anchored to a first vertebra in a subject, the first bone anchor 10comprising a first bone fastener 15 attached to a first rod housing 20.A rigid spinal rod 25 is seated in the first rod housing 20 to restricttranslation of the rigid spinal rod 25 relative to the first bone anchor10. The rigid spinal rod 25 is seated in the rod housing 35 of a secondbone anchor 30, anchored to a second vertebra in the subject, so as torestrict translation of the rigid spinal rod 25 relative to the secondbone anchor 30. A compressible spinal connector 40 is connected to thesecond bone anchor 30 and anchored to a third vertebra in the subject.The compressible spinal connector 40 has a modulation mechanism 45 formodulating either the tension on the compressible spinal connector 40 orits resistance to compression (or both). The modulation occurs inresponse to a remote signal. Consequently modulation of the tensionand/or resistance to compression does not require access to the device 5through the patient's tissues, and may be performed post-operatively.The remote signal may be, for example, an electromagnetic signal. Aspecific example of the remote signal is a spinning magnetic field.

FIG. 1 illustrates an example of a vertebral fixation system 5 of thetype that is used with the devices and methods described in thisdisclosure. By way of example, the illustrated vertebral fixation system5 is a screw-and-rod construct adapted for implantation along theposterior aspect of the human spinal column. The vertebral fixationsystem 5 includes a pair of elongate rods (50 a, 50 b) dimensioned tospan multiple vertebral levels, a plurality of threaded bone anchors 55,a plurality of hook-type bone anchors 60, and a plurality of transverseconnectors 65 dimensioned to rigidly engage each of the elongate rods(50 a, 50 b) so as to hold each rod (50 a, 50 b) in place relative tothe other. The transverse connectors 65 may be provided as fixedconnectors or adjustable connectors, in any quantity that is required bythe surgeon performing the implantation surgery. Proximal bone anchors70 are provided at the proximal (cephalad) terminus of the assembly 5.Distal bone anchors 75 are provided at the distal (caudal) terminus ofthe assembly 5. It is contemplated that any of the examples of boneanchors and other transition assemblies described herein may besubstituted for the cephalad bone anchors 70 and/or caudal bone anchors75 which are traditionally rigid and identical to the other bone anchorsused throughout the construct 5. It is also contemplated that theexamples of flexible or compressible transition segments 80 describedherein may replace existing hardware at the cephalad and/or caudalterminus of the vertebral fixation system 5 such that there is noadditional surgical footprint realized. It is further contemplated thatthe examples of flexible or compressible transition segments 80described herein may augment existing hardware at the cephalad and/orcaudal terminus of the vertebral fixation system 5 such that there isadditional added surgical footprint realized. This may be moreapplicable with the various embodiments that can be installed withminimal disruption of additional muscle tissue and/or ligamentstructure. Finally, as previously noted junctional disease or failurecan be a problem at either the cephalad or caudal terminus (or both) ofvertebral fixation systems 5. Therefore, although the various examplesdisclosed herein may be described in terms of cephalad terminus andproximal joint disease (for ease of disclosure) it is to be understoodthat any of the example embodiments are also applicable and may be usedat the caudal terminus of the vertebral fixation system 5 withoutdeviating from the scope of this disclosure. According to one example, aspinal fixation construct 5, like that shown in FIG. 1, is applied tothe spinal levels to be fixed. Above the fixed levels a soft-zone iscreated by applying non-rigid support elements 85 such as tethers oradjustable rods that limit some motion and reduce stress, to the levelsof the soft-zone and above, while not inhibiting all motion. The tensionapplied to the support elements 85 in the soft zone can be adjustedpost-operatively and non-invasively to account for changing dynamics inthe body, or for any other reason deemed desirable.

The components in the system 5 are constructed from one or morenon-absorbable biocompatible materials. Specific examples of suchsuitable materials include titanium, alloys of titanium, steel, andstainless steel. Parts of the system 5 could conceivably be made fromnon-metallic biocompatible materials, which include aluminum oxide,calcium oxide, calcium phosphate, hydroxyapatite, zirconium oxide, andpolymers such as polypropylene. Interference with the spinning magneticfield can be reduced by constructing one or more portions of the system5 from a nonmagnetic or weakly magnetic material. Specific examples ofsuch nonmagnetic non-absorbable biocompatible material include titanium,alloys of titanium, aluminum oxide, calcium oxide, calcium phosphate,hydroxyapatite, zirconium oxide, and polymers such as polypropylene.Examples of weakly magnetic materials include paramagnetic materials anddiamagnetic materials. In a specific embodiment, the weakly magneticmaterial is austenitic stainless steel.

The first, second, and third vertebrae may be adjacent or non-adjacentto one another, in any combination. Thus it is contemplated that thefirst vertebra will be adjacent to the second, which will be adjacent tothe third; the first vertebra will be nonadjacent to the second, whichwill be adjacent to the third; the first vertebra will be nonadjacent tothe second, which will be nonadjacent to the third; and that the firstvertebra will be adjacent to the second, which will be nonadjacent tothe third.

According to one example the non-rigid support structure 5 is createdthrough the application of a compressible spinal connector 40 in theform of one or more tether assemblies 95, such as those shown in theexemplary embodiment in FIG. 3. In the tether assembly 95 the modulationmechanism 45 is an adjustable tensioner 100 configured to vary thetension on a flexible tether 97. This will control the tension betweenthe second bone anchor 30 and the third vertebra.

The tethers 97 may be attached between the fixation hardware 5 and thesoft-zone (e.g. one or more non-fixed levels above), and/or directlybetween the bone elements of one or more fixed levels and the soft-zone,and/or between two or more of the non-fixed levels in the soft-zone. Thetether 97 may be formed of any material suitable for medical use. Forexample, the tether 97 may be made from allograft tendon, autografttendon, braided, woven, or embroidered polyethylene, braided, woven, orembroidered polyester, polyether ether ketone (PEEK), orpolyetherketoneketone (PEKK). In some instances the tether 97 may beformed of elastic material. FIG. 3 depicts multiple tethers 97 appliedto the spine in the soft zone and connected to the fixation construct 5by different connectors (e.g. an adjustable tension tether-rodconnector, adjustable tension cross-connector, and turnbuckle (FIG.7)—these may be collectively referenced herein simply as adjustabletension connectors 215). It will be appreciated that while shown in usetogether, either of these connectors 215 may also be used on their own.and in any configuration desired. In use, once the tethers 97 areconnected to the fixation construct 5 and the tether 97 coupled to thedesired bone structure (or other bone connection element) the tension onthe tether 97 can be adjusted. Later, the tension on the tether 97 canbe adjusted post-operatively as desired using an external device 155.

A specific example of the adjustable tensioner 100 is a turnbuckle 105comprising a threaded first end coupler 110, a second end coupler 115,and a rotatable magnet 120 that rotates in response to a spinningmagnetic field and that is connected to the threaded first end coupler110 to cause the threaded first end coupler 110 to rotate about itslongitudinal axis when the rotatable magnet 120 rotates. An embodimentof such a turnbuckle 105 is shown in FIG. 7. The illustration is across-section of the turnbuckle 105, showing a cylindrical magnet 125oriented to rotate around its longitudinal axis when exposed to aspinning magnetic field in the right orientation. The threaded first endcoupler 110 in the illustration is a hook 130 with a threaded shank 135.The threaded shank 135 runs through a threaded channel 140 in thehousing 145 of the turnbuckle 105, which translates rotation of theshank 135 into translation of the hook 130. Alternatively, the magnet125 itself may contain one or more threaded passages 150 that areengaged with the threaded shank(s) 135. As a result the hook 130 can beextended or retracted by rotating the shank 135. In some embodiments ofthe turnbuckle 105 both end couplers (110, 115) are hooks (130 a, 130 b)with threaded shanks 135, the threads oriented such that when therotatable magnet 125 rotates in a given direction the two hooks (130 a,130 b) translate in opposite directions (i.e., they either converge ordiverge along their shared longitudinal axis). When the hooks (130 a,130 b) are caused to converge, it will increase tension on the connectedspinal physiology.

As pictured in FIG. 7, the turnbuckle 105 can be attached to the spinousprocess or lamina of two adjacent vertebrae (either directly, or viatethers looped around the lamina or spinous process), and the tensionbetween the vertebrae can be adjusted post-operatively andnon-invasively using the external adjustment device 155 to rotate theturnbuckle magnet 120. Though shown only across a single level,turnbuckles 105 could be used at multiple levels. According to oneexample the turnbuckles 105 can be used selectively to set the tensiondifferently at each level. By way of example, the tension can start outhigher closest to the fixed spinal levels, and be sequentially decreasedover a series of levels through the soft-zone. In some embodiments ofthe system a pair of turnbuckles (105 a, 105 b) is used bilaterally andcoupled to tethers 97 looped around the lamina and the superior andinferior coupled vertebrae. A distraction device 160 is also positionedbetween the spinous processes of the same level. It is contemplated thatthe distraction device 160 could use a magnetically driven expansiondevice (such as one utilizing a lead screw coupled to a magnet, similarto that described below, to create linear expansion). This way, bothflexion and extension could be effectively controlled, and adjustedpost-operatively. Taking it a step further, the addition of a rotatableelement 220 within the disc to allow the vertebrae to rotate relative toeach other, could facilitate scoliosis correction in both the sagittaland coronal planes using the adjustable turnbuckle 105 and distractiondevices 160.

Another embodiment of the adjustable tensioner 100 is a spool 165 aboutwhich the flexible tether 97 is wound, and wherein rotation of a spoolmagnet 170 drives rotation of the spool 165. An example of such anembodiment is shown in FIG. 4, and the remaining description in thisparagraph refers to this example. The spool 165 is connected to a body175 having a rod passage 180 that couples to the rod 50. A setscrew 205or other element may be used to lock the body 175 to the rod 50. Thespool 165 is rotatably connected to the body 175. The tether 97 isattached to the spool 165 such that when the spool 165 rotates thetether 97 is wound up on the spool 165 to create tension. A first end185 of the tether 97 may be attached to the spool 165 with the secondend 190 being otherwise attached to the target bone, another adjustableconnector, or to itself (e.g. creating a loop that can be attached tothe target bone, or both ends of the tether (185, 190) may be coupled tothe spool 165 creating a loop to attach to or around the bone such thatboth ends of the tether (185, 190) are spooled up together). The spoolmagnet 170 can be driven by application of a magnetic field to rotatethe spool 165. The spool magnet 170 may be a single cylindrical magnetpoled north and south across its diameter to form two 180 degreesectors, as in FIG. 4. Alternatively, a quadrupole or multipole magnetmay be used. The spool 165 may include a locking mechanism 195, such asa spool 165 and ratchet mechanism 200, to maintain the tension applied.The locking mechanism 195 may be externally controlled similar to thespool magnet 170 such that it can be locked and unlocked if adjustmentis needed. According to one embodiment, the locking mechanism 195 may bea set screw 205 situated to inhibit rotation of the spool 165 whenengaged. The set screw 205 may be a magnetically driven set screw 225,oriented such that the external drive controller 155 can be positionedto drive only one of the set screw 205 and drive magnet 230, and thenprepositioned to drive the other. Alternatively, a locking pin or shaftcould be advanced with the set screw 205 to inhibit rotation of thespool 165. In an alternative embodiment shown in FIG. 5, the magnet 230may drive a gear assembly 380 that rotates two opposing ratchet wheelsthrough 385 which the tether 97 is passed.

Another embodiment of the compressible spinal connector 40 is adampening rod 235. The dampening rod 235 is a rod that is bothexpandable and compressible, and the resistance to expansion andcompression is controlled by means of the modulation mechanism 45. Themodulation mechanism 45 in this embodiment may take the form of afriction brake 240. The dampening rod 235 accommodates dynamic travel orlength adjustment of the rod 235 between the fixed connectors 390. Thefriction brake 240 can include a set screw 205 that is itself magnetic,or connected to a magnet (“brake magnet”) 245 that may be controllablevia an external adjustment device 155. The degree of tension and supportprovided by the dampening rod 235 can be controlled by increasing ordecreasing friction with the set screw 205. Some embodiments of thefriction brake 240 can also lock down the rod 235 entirely, to preventany expansion or compression, should it later become necessary to fixone or more levels in the soft-zone. An embodiment of the dampening rod235 is shown in FIG. 2. In that embodiment, each rod 235 has a widerbell region 395 and a narrower tail region 400. The tail 400 is aboutthe diameter of an ordinary spinal rod. The bell 395 is open on theinside, and is dimensioned to accommodate a spinal rod (or the tail ofanother dampening rod 235). As shown in FIG. 2, the friction brake 240may be, for example, a set screw 205 in a threaded channel 405positioned to exert compressive force on a spring 250, said spring 250positioned to exert compressive force against both the compression andexpansion of the dampening rod 235. The illustration in FIG. 2 shows thespring 250 positioned to exert compressive force on the tail portion 400of an adjacent dampening rod 235. The spring 250 in FIG. 2 is a wavespring 410, although other kinds of springs (e.g., helical) arecontemplated as well. The set screw 205 may be magnetic (225), orcoupled to a magnet, such that a rotating magnetic field in the properorientation will cause the set screw 225 to rotate, either increasing ordecreasing the compressive force that exerts the friction.

A telescoping rod 255 may also be employed in the system. Thetelescoping rod 255 may be implanted at levels above a fixationconstruct 5 in patients that are at high risk of developing PJK or otheradjacent segment diseases. The rods 255 may be implanted as aprophylactic and used if needed to extend the length for pain relief. Anexample of the telescoping rod 255 is shown in FIG. 6. As shown in thatfigure, the telescoping spinal rod 255 comprises a rod magnet 260configured to rotate when exposed to a spinning magnetic field and causethe telescoping spinal rod 255 to either extend or collapse depending onthe direction of the spinning magnetic field. The rod magnet 260 may bea cylindrical permanent magnet (such as a ferrimagnet), but may beanother type of magnet. A first elongate element 265 contains a cavity270, into which fits a second elongate element 275. There may be adynamic seal between the first 265 and second elongated elements 275, toensure that no bodily fluids enter the construct. A thrust bearing maybe included to reduce friction between the spinning magnetic element andthe housing. The second elongate element 275 has an internally threadedregion 280 that is engaged to a lead screw 285 coupled to rotate whenthe rod magnet 260 rotates, and comprising an externally threaded region290, such that rotation of the lead screw 285 causes the second elongateelement 275 to translate relative to the first elongate element 265.Thus, when the magnet 260 rotates, the lead screw 285 will also rotate.The threaded interface between the lead screw 285 and the secondelongated element 275 will then cause the second elongated element 275to translate, with the translational direction being dependent upon therotational direction of the magnet 260. The threads on the internallythreaded region 280 of the first elongate element 265 may be integral,or they may be on the inner surface of another structure, such as athreaded nut.

Whenever the adjustment mechanism is actuated by the rotation of amagnet 120, as a safety precaution, a magnetic immobilization plate 295may be positioned sufficiently close to the rotatable magnet 120 tocause the rotatable magnet 120 to adhere to the immobilization plate 295in the absence of a strong external magnetic field. The magneticimmobilization plate 295 will hold the rotating magnet 120 in position,preventing it from rotating, until a stronger magnetic field is applied,such as the rotating magnetic field that is used to adjust themodulation mechanism 45. Like the rotating magnet 120, theimmobilization plate 295 may be constructed from a suitable magneticmaterial, such as a ferromagnetic material. The immobilization plate 295may be used on its own, or in combination with a locking mechanism 195as described above.

A specific embodiment of the system is shown in FIG. 3. In thatembodiment, the system 5 comprises a third bone anchor 300 (comprising athird bone fastener 305 and a third rod housing 310) anchored to thefirst vertebra, and a fourth bone anchor 320 (comprising a fourth bonefastener 330 and a fourth rod housing 340) anchored to the secondvertebra. A second rigid spinal rod is 25 b seated in the third rodhousing 310 and the fourth rod housing 340, running roughly parallel tothe first rod 25 a. A first flexible tether 97 a is at least partiallywrapped around a spinous process of the third vertebra and connected toboth of the first 25 a and second rigid spinal rods 25 b to exerttension between the third vertebra and the first 25 a and second rigidspinal rods 25 b. The adjustable tether assembly 95 comprises a secondflexible tether 97 b encircling the spinous process of the thirdvertebra and a spinous process of a fourth vertebra, and a tensioner 350connected to the first 25 a and second 25 b rigid spinal rods, thetensioner 350 comprising a first magnet 355 mounted to rotate inresponse to a spinning magnetic field, and a tether connection 360configured to increase or decrease the tension on the second flexibletether 97 b depending on the direction of rotation of the first magnet355. The tensioner 350 extends or retracts the tether connection 360depending on the direction of the spinning magnetic field, eitherreducing or increasing the tension respectively.

The system may be bilateral, in which the network of bone anchors androds is present on either side of the spine. Such a bilateral system maycomprise a second rigid spinal rod 25 b seated in an additional rodhousing 365 of an additional bone anchor 370 that is anchored in atleast one of the first and second vertebrae. As shown in FIG. 1, one ormore transverse connectors 65 fastened to the first rigid spinal rod 25a and the second rigid spinal rod 25 b may be present for additionalstability.

Methods of using the system 5 to fix the relative positions of a firstvertebra and a second vertebra in a subject are provided. In a generalembodiment the method comprises anchoring a first bone anchor 10 to thefirst vertebra, the first bone anchor 10 comprising a first bonefastener 15 attached to a first rod housing 20; seating a rigid spinalrod 25 a in the first rod housing 20 to restrict translation of therigid spinal rod 25 a relative to the first bone anchor 10; anchoring asecond bone anchor 30 to the second vertebra, the second bone anchor 30comprising a second bone fastener 33 attached to a second rod housing35; seating the second rigid spinal rod 25 b in the second rod housing35 to restrict translation of the rigid spinal rod 25 b relative to thesecond bone anchor 30; connecting a compressible spinal connector 40 tothe second bone anchor 30, the compressible spinal connector 40comprising a modulation mechanism 45 for modulating at least one of thetension on the compressible spinal connector 40 or the resistance tocompression of the compressible spinal connector 40, wherein saidmodulation 45 occurs in response to a remote signal; anchoring thecompressible spinal connector 40 to a third vertebra in the subject; andtransmitting the remote signal to the modulation mechanism 45post-operatively, to cause said modulation to occur. The system 5 mayhave any of the components and arrangements described above. Thecompressible spinal connector 40 can be any described as suitable forthe system above, including any of the described embodiments of thetether assembly 95, dampening rod 235, and telescoping rod 255.

An example of an external adjustment device 155 used to non-invasivelydrive the adjustment mechanisms on the various implants described hereinis represented in FIG. 9. The external adjustment device 155 isconfigured for placement on or adjacent to the skin of the subject andappropriately aligned with the magnet to be activated, and includes atleast one drive magnet 230 configured for rotation. The externaladjustment device 155 further comprising a motor 375 configured torotate the at least one drive magnet 230, whereby rotation of the atleast one drive magnet 230 of the external adjustment device 155effectuates rotational movement of one or more magnets (e.g., rotatablemagnet in the turnbuckle, magnetic set screw, etc). As shown in FIG. 9,the external adjustment device 155 may have two magnets (230 a, 230 b).The two magnets (230 a, 230 b) may be configured to rotate at the sameangular velocity. They may also be configured to each have at least onenorth pole and at least one south pole (i.e., a dipole magnet), and theexternal adjustment device 155 is configured to rotate the first drivemagnet 230 a and the second drive magnet 230 b such that the angularlocation of the at least one north pole of the first drive magnet 230 ais substantially equal to the angular location of the at least one southpole of the second drive magnet 230 b through a full rotation of thefirst 230 a and second 230 b drive magnets. More complex systemsinvolving quadrupole and multipole drive magnets are also contemplated,as is the use of one or more electromagnets. The foregoing descriptionillustrates and describes the processes, machines, manufactures,compositions of matter, and other teachings of the present disclosure.Additionally, the disclosure shows and describes only certainembodiments of the processes, machines, manufactures, compositions ofmatter, and other teachings disclosed, but, as mentioned above, it is tobe understood that the teachings of the present disclosure are capableof use in various other combinations, modifications, and environmentsand is capable of changes or modifications within the scope of theteachings as expressed herein, commensurate with the skill and/orknowledge of a person having ordinary skill in the relevant art. Theembodiments described hereinabove are further intended to explaincertain best modes known of practicing the processes, machines,manufactures, compositions of matter, and other teachings of the presentdisclosure and to enable others skilled in the art to utilize theteachings of the present disclosure in such, or other, embodiments andwith the various modifications required by the particular applicationsor uses. Accordingly, the processes, machines, manufactures,compositions of matter, and other teachings of the present disclosureare not intended to limit the exact embodiments and examples disclosedherein. Any section headings herein are provided only for consistencywith the suggestions of 37 C.F.R. §1.77 and related laws or otherwise toprovide organizational queues. These headings shall not limit orcharacterize the invention(s) set forth herein.

The following is claimed:
 1. A system for spinal fixation, the systemcomprising: a first bone anchor, anchored to a first vertebra in asubject, the first bone anchor comprising a first bone fastener attachedto a first rod housing; a rigid spinal rod seated in the first rodhousing to restrict translation of the rigid spinal rod relative to thefirst bone anchor; a second bone anchor, anchored to a second vertebrain the subject, the second bone anchor comprising a second bone fastenerattached to a second rod housing, wherein the rigid spinal rod is seatedin the second rod housing to restrict translation of the rigid spinalrod relative to the second bone anchor; and a compressible spinalconnector, connected to the second bone anchor, and anchored to a thirdvertebra in the subject, the compressible spinal connector comprising amodulation mechanism for modulating at least one of: the tension on thecompressible spinal connector or the resistance to compression of thecompressible spinal connector, wherein said modulation occurs inresponse to a remote signal.
 2. The system of claim 1, wherein theremote signal is a spinning magnetic field.
 3. The system of claim 1,wherein at least one of the following is at least partially composed ofa non-absorbable biocompatible material: the first bone anchor, thesecond bone anchor, the rigid spinal rod, and the compressible spinalconnector.
 4. The system of claim 1, wherein the remote signal is aspinning magnetic field, and wherein at least one of the following is atleast partially composed of a non-absorbable biocompatible material thatis either non-magnetic or weakly magnetic: the first bone anchor, thesecond bone anchor, the rigid spinal rod, and the compressible spinalconnector.
 5. The system of claim 1, wherein the compressible spinalconnector is a tether assembly.
 6. The system of claim 5, wherein themodulation mechanism is an adjustable tensioner configured to vary thetension on a flexible tether between the second bone anchor and thethird vertebra.
 7. The system of claim 6, wherein the adjustabletensioner is a turnbuckle comprising a threaded first end coupler, asecond end coupler, and a rotatable magnet that rotates in response to aspinning magnetic field and that is connected to the threaded first endcoupler to cause the threaded first end coupler to rotate about itslongitudinal axis when the rotatable magnet rotates.
 8. The system ofclaim 6, wherein the adjustable tensioner comprises a spool about whichthe flexible tether is wound, and wherein rotation of a spool magnetdrives rotation of the spool.
 9. The system of claim 6, wherein theadjustable tensioner comprises a locking mechanism configured tomaintain tension on the flexible tether when engaged.
 10. The system ofclaim 5, wherein the flexible tether is constructed of a non-absorbablebiocompatible material.
 11. The system of claim 1, wherein thecompressible spinal connector is a dampening spinal rod that iscompressible and expandable.
 12. The system of claim 11, the dampeningspinal rod comprising: an elongate rigid portion for insertion into abone anchor; a flared portion for receiving a terminal end of a secondspinal rod, the flared portion comprising a rod cavity of sufficientdiameter to accept the second spinal rod.
 13. The system of claim 11,wherein the modulation mechanism is a friction brake configured to varythe resistance of the dampening rod to compression and tension.
 14. Thesystem of claim 13, wherein the friction brake comprises a set screw ina threaded channel positioned to exert compressive force on a spring,said spring positioned to exert compressive force against both thecompression and expansion of the dampening rod.
 15. The system of claim14, wherein the set screw is magnetic and rotates in the threaded,channel in response to a spinning magnetic field.
 16. The system ofclaim 1, wherein the compressible spinal connector is a telescopingspinal rod positioned within the second rod housing.
 17. The system ofclaim 16, wherein the telescoping spinal rod comprises: a rod magnetconfigured to rotate when exposed to a spinning magnetic field and causethe telescoping spinal rod to either extend or collapse depending on thedirection of the spinning magnetic field; a first elongate elementcontaining a cavity; and a second elongate element dimensioned to atleast partially fit within the cavity, and having an internally threadedregion; wherein the modulation mechanism comprises a lead screw coupledto rotate when the rod magnet rotates, and comprising an externallythreaded region engaged to the internally threaded region of the secondelongate element, such that rotation of the lead screw causes the secondelongate element to translate relative to the first elongate element.18. The system of claim 1, wherein a second rigid spinal rod is seatedin an additional rod housing of an additional bone anchor that isanchored in at least one of the first and second vertebrae.
 19. Thesystem of claim 18, wherein a transverse connector is fastened to thefirst rigid spinal rod and the second rigid spinal rod.
 20. The systemof claim 1, comprising: a third bone anchor comprising a third bonefastener and a third rod housing, anchored to the first vertebra; afourth bone anchor, comprising a fourth bone fastener and a fourth rodhousing, anchored to the second vertebra; a second rigid spinal rodseated in the third rod housing and the fourth rod housing; and a firstflexible tether at least partially wrapped around a structure of thethird vertebra and connected to both of the first and second rigidspinal rods to exert tension between the third vertebra and the firstand second rigid spinal rods; wherein the compressible spinal connectoris an adjustable tether assembly, the adjustable tether assemblycomprising: a second flexible tether encircling the structure of thethird vertebra and a spinous process of a fourth vertebra; and atensioner connected to the first and second rigid spinal rods, thetensioner comprising a first magnet mounted to rotate in response to aspinning magnetic field, and a tether connection configured to increaseor decrease then tension on the second flexible tether depending on thedirection of rotation of the first magnet.